1. Field of the Invention
The present invention relates to a method of manufacturing a MONOS nonvolatile semiconductor memory device. In particular, the present invention relates to a method of manufacturing a nonvolatile semiconductor memory device using a metal oxide as a block insulating film.
2. Description of the Related Art
Recently, a MONOS memory using an insulating film such as a silicon nitride film as a charge storage layer has been developed for one of the future nonvolatile semiconductor memory device. In general, the MONOS memory is configured to a structure that a charge storage layer is formed through a tunnel insulating film on a semiconductor substrate, and a block insulating film is formed on the charge storage layer, and further, a control gate electrode is formed on the block insulating film. A write/erase operation of a memory cell is carried out by applying high electric field to the tunnel insulating film. In this case, the block insulating film requires high insulation properties in order to prevent carrier which is trapped in the charge storage layer from leaking through the block insulating film during the data retention or Program/Erase operation.
In contrast, a high-dielectric constant insulating film containing a metal oxide (High-k film: The dielectric constant is typically 7 or more, which is higher than the value of silicon nitride) is effective to reduce a leakage current. Because the physical film thickness can be gotten thick without increasing EOT (Equivalent Oxide Thickness). Therefore, the high-dielectric constant insulating film is effective as the block insulating film of the MONOS memory. For example, a MANOS structure using an Al2O3 film (dielectric constant: 10) as the block insulating film is given (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2007-282859).
However, in fact, if the High-k film was applied for the block insulating film of the MONOS memory, it never realizes the sufficient dielectric characteristics corresponding to its physical property. For this reason, there is a problem that desired data retention and write/erase characteristics are not obtained.